This application claims all benefits accruing under 35 U.S.C. xc2xa7 119 from an application entitled Video Stabilizing Circuit For Display Apparatus earlier filed in the Korean Industrial Property Office on Mar. 17, 1998, and there duly assigned Serial No. 98-9112 by that Office.
1. Technical Field
The present invention relates to a video stabilizing circuit for a display apparatus. More specifically, the present invention relates to a video stabilizing circuit for a display apparatus, in which the clamp levels of R, G and B chromatic signals of the video output sections are controlled by feeding back the output currents of respective R, G and B terminals, so that real time responses can be carried out as against the fluctuations of video output load, thereby obtaining high brightness displays.
2. Related Art
Generally, in a display apparatus, the white balance is adjusted by varying the gains of R, G and B chromatic signals which are inputted from a computer system. Thus, dark and bright portions provide clear contrasts.
Further, a cathode ray tube which is used in the display apparatus requires a particular range of brightness value. That is, cut-off voltages of the cathode ray tube are set in accordance with the bias voltages of the R, G and B chromatic signals of the cathode ray tube. The fluctuations of the bias voltages, which fluctuate in accordance with the cut-off voltages thus set, cause fluctuations in brightness.
Recently, display devices have developed to the point where they perform multi-media functions, thereby requiring high brightness and high clarity. Accordingly, many kinds of products having the required functions have appeared. However, one of the technical problems in these products is as follows. When high and low beams are displayed, the video output bias voltages fluctuate, and therefore the brightness of the back-raster fluctuates.
The term xe2x80x9cluminancexe2x80x9d (brightness) will be briefly described. This term refers to the brightness information for the picture which is to be transmitted to the cathode ray tube for display on the screen. If the brightness is too high, a large anode current flows in the cathode ray tube so as to generate a high voltage. Because of this high voltage, an over-load is imposed on the high voltage driving part. As a result, the switching device of a horizontal output circuit is damaged, or a xe2x80x9cdomingxe2x80x9d phenomenon occurs in the shadow mask tube so as to cause color spreading in the picture.
One of the methods for preventing this phenomenon has been as follows. An automatic beam limit (ABL) circuit is employed to restrict the brightness when high beams and low beams of video signals are displayed.
As another method, in order to adjust the brightness, a switch or a volume switch is used, or a brightness adjusting command is inputted through the keyboard. In this manner, the first grid voltage or the cathode clamp level in the electron gun is varied, thereby adjusting the brightness.
That is, the greater the voltage between the cathode and the first grid, the smaller the amount of the electrons emitted from the cathode, thereby lowering the brightness. On the other hand, the lower the voltage between the cathode and the first grid, the greater the amount of the electrons emitted from the cathode, thereby intensifying the brightness.
However, in such a display apparatus, when high beams and low beams are displayed, the video output levels have to be adjusted manually to adjust the brightness, and this is a troublesome task.
The present invention is intended to overcome the above described disadvantages.
Therefore it is an object of the present invention to provide a video stabilizing circuit for a display apparatus, in which the clamp levels of R, G and B chromatic signals are controlled by feeding back the output currents of R, G and B terminals, so that real time responses can be made as against the fluctuations of the video output load, thereby making a high brightness display possible.
In achieving the above object, the circuit according to the present invention is characterized in that: the output currents of R, G and B terminals are fed back from video pre-amplifiers to sense the magnitudes of the beam currents by a clamp level control section; and the potential of a cut-off control terminal of the video pre-amplifier is varied in such a manner that, if the sensed beam current is large, the bias voltage of the chromatic signals is made small, and if the beam current is small, the bias voltage of the chromatic signals is made large.